Wear- and seizing-resistant roll for hot rolling and method of making the roll

ABSTRACT

PCT No. PCT/JP94/00520 Sec. 371 Date Feb. 3, 1995 Sec. 102(e) Date Feb. 3, 1995 PCT Filed Mar. 30, 1994 PCT Pub. No. WO94/22606 PCT Pub. Date Oct. 13, 1994.A wear- and seizing-resistant roll for hot rolling has a composition consisting essentially, by weight, of 2.0-4.0% of C, 0.5-4.0% of Si, 0.1-1.5% of Mn, 1.0-7.0% of Cr, 2.0-10.0% of Mo, 2.0-8.0% of V and balance of Fe and inevitable impurities; and has a metal structure comprising a matrix substantially comprising martensite, bainite or pearlite, 0.5-5% in area ratio of graphite, 0.2-10% in area ratio of MC carbides and 40% or less in area ratio of cementite. The roll of the present invention is suitable for work roll in the latter stand of a finishing train of a hot strip mill.

TECHNICAL FIELD

The present invention relates to a wear- and seizing-resistant roll forhot rolling which s required to have higher wear resistance and abilityto withstand abnormal rolling operations, and particularly, to a wear-and seizing-resistant roll for hot rolling suitable for a work roll inthe latter stand of a finishing train of a hot strip mill.

BACKGROUND ART

Conventionally, rolls having an outer layer of grain cast iron have beenused in the latter strand of a finishing train of a hot strip mill. Whengrain rolls meet abnormal draw rolling, the grain rolls suffer fromlittle seizing of rolled material as well as little occurring orextending of cracks, because the grain roll, in general, is excellent inseizing resistance. However, the grain roll is fairly inferior in wearresistance to a compound roll having an outer layer of high-speed steelmaterial, which has is recently come to be widely used. Although thehigh-speed steel roll is excellent in wear resistance, such roll issusceptible to seizing of rolled material by abnormal draw rolling,resulting in ocurring or extending cracks due to the stressconcentration at seizing portion by high pressure from back-up rolls orthe rolled material.

It has been known that crystallization or precipitation of hard carbidessuch as MC, M₂ C, etc is effective for improving wear resistance of aroll. Also, it has been known that crystallization of graphites which asa solid lubricant can improve seizing resistance of a roll. However, V,Mo, W which are hard carbide-forming elements are also white castiron-forming elements. Therefore, it has been unable to crystallizing asuitable amount of graphite in high-speed steel roll containing a largeamount of these white cast iron-forming elements to allow hard carbidesand graphites to coexist.

To solve this problem, various attempts have been made. JP-B-60-23183discloses a tough, wear-resistant roll for rolling mill made of a castironing a composition consisting of 2.2-2.9% of C, 0.8-1.5% of Si,0.5-1.0% of Mn, 0.1% or less of P, 0.1% or less of S, 3.8-4.8% of Ni,1.7-2.5% of Cr, 0.4-1.0% of Mo and balance substantially consisting ofFe. The roll has a structure comprising a matrix of martensite and/orbainite, carbides having an area ratio of 10-30% and graphites having anarea ratio of 0.5-3%. The Shore hardness of the roll is 70-85. The rollof JP-B-60-23183, however, is insufficient in in wear resistance becauseof a small amount of carbides.

JP-A-61-26758 discloses a seizing-resistant compound roll having anouter layer of a composition consisting, by weight, of 1.0-2.0% of C,0.2-2.0% of Si, 0.5-1.5 of Mn, 3.0% or less of Ni, 2-5% of Cr, 3-10% ofMo, 4.0% or less of V 0.1-0.6% of S and balance substantially consistingof Fe. In this roll, seizing resistance is intended to be improved byforming MnS, etc. However, it is now known that graphite is moreeffective than MnS for improving seizing resistance.

JP-A-2-30730 disclose a wear-resistant cast iron for use in a roll forhot or cold rolling, having a composition consisting, by weight, of2.5-4.0% of C. 2.0-5.0% of Si, 0.1-1.5% of Mn, 3-8% of Ni, 7% or less ofCr, 4-12% of Mo, 2-8% of V and balance consisting of Fe and impurities.This cast iron contains graphites and hard carbides such as MC, M₂ C, M₆C, M₄ C₃, etc. in an area ratio of 20% or less. In this cast iron, anSi-containing inoculant such as Fe--Si alloy, etc. is added into a meltof a casting material to crystallize graphite. Specifically, in Example1, an Fe--Si alloy is inoculated into a melt in a ratio of 0.3% based onSi to obtain a casting product in which the area ratio of graphite is 2%and the ratio of the area of hard carbides to the area of total carbidesis 85%.

In case of a high-speed steel roll, however, it has been found thatgraphite does not crystallize in a sufficient amount by the inoculationmethod disclosed in JP-A-2-30730, because a sufficient effect ofinoculation cannot be obtained by merely adding an inoculant into a meltat tapping of the melt.

It is difficult to insure a sufficient crystallization of graphite in anouter layer, namely, in a high-speed steel roll disclosed in WO88/07594, namely, a wear-resistant compound roll comprising an outerlayer of an iron-based alloy consisting, by weight, of 1.5-3.5% of C,0.3-3.0% of Si, 0.3-1.5% of Mn, 2-7% of Cr, 9% or less of Mo, 20% orless of W, 3-15% of V and balance substantially consisting of Fe, and asteel shaft metallurgically bonded to the outer layer; and produced by ashell casting method.

Accordingly, an object of the present invention is to solve the aboveproblem and provide a graphite-containing, high-speed steel roll for hotrolling excellent in both wear resistance and seizing resistance.

DISCLOSURE OF INVENTION

The wear- and seizing-resistant roll for hot rolling of the presentinvention has a composition consisting essentially, by weight, of2.0-4.0% of C, 0.5-4.0% Si, 0.1-1.5% of Mn, 1.0-7.0% of Cr, 2.0-10.0% ofMo, 2.0-8.0% of and balance of Fe and inevitable impurities, and has ametal structure comprising a matrix, 0.5-5% in area ratio of graphite,0.2-10% in area ratio of MC carbides and 40% or less in area ratio ofcementite.

The wear- and seizing-resistant compound roll for hot rolling accordingto the present invention comprises an outer layer of a wear- andseizing-resistant iron-based alloy and a steel shaft metallurgicallybonded to the outer layer, the iron-based alloy having a compositionconsisting essentially, by weight, of 2.0-4.0% of C, 0.5-4.0% of Si,0.1-1.5% of Mn, 1.0-7.0% of Cr, 2.0-10.0% of Mo, 2.0-8.0% of V andbalance of Fe inevitable impurities, and having a metal structurecomprising a matrix, 0.5-5% in area ratio of graphite, 0.2-10% in arearatio of carbides and 40% or less in area ratio of cementite.

The method of producing the wear- and seizing-resistant compound rollfor hot rolling according to the present invention is characterized insupplying an Si-containing inoculant at least in a vicinity of thebonding of the melt for the outer layer and the steel shaft.

Preferably, the method of producing the wear- and seizing-resistantcompound roll for hot rolling according to the present inventioncomprises the steps of introducing the steel shaft concentrically intoan inner space of a composite mold comprising a refractory moldsurrounded by an induction heating coil and a cooling mold providedunder the refractory mold concentrically therewith; pouring a melt ofthe m-based alloy into a space between the steel shaft and the compositemold; keeping the melt at a temperature between a primarycrystal-crystallizing temperature and a temperature 100° C. higher thanthe primary crystal-crystallizing temperature un heating with stirringwhile sealing the surface of the melt by flux; moving the steel shaftdownward concentrically with the composite mold to bring the melt intocontact with the cooling mold hereby solidifying the melt to bond to thesteel shaft so that the toter layer is continuously formed on the steelshaft body, during the formation of the outer layer an Si-containinginoculant is injected by means of wire-injection method into a vicinityof tile bonding portion of the melt and the steel shaft to crystallizeparticles in a sufficient amount.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic -sectional view showing an apparatus for producingthe wear- and seizing-resistant compound roll for hot rolling accordingto the invention by a shell casting method;

FIG. 2 is a microphotograph (×100) showing the metal structure of thetest roll No. 2 in Example 1 after diamond polishing;

FIG. 3 is a microphotograph (×100) showing the metal structure of thetest roll No. 2 in Example 1 after etching treatment with picric acid;

FIG. 4 is a microphotograph (×100) showing the metal structure of thetest roll No. 2 in Example 1 after electrolytic etching;

FIG. 5 is a schematic view showing a rolling wear test apparatus used inExample 2; and

FIG. 6 is a schematic view of a frictional-heat shock test apparatusused in Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION 1! Wear- and seizing-resistantrot for hot rolling

(a) Metal structure

The wear- and seizing-resistant roll for hot rolling of the presentinvention has the following metal structure.

(1) The content of the graphite particles is 0.5-5% by area ratio. Asufficient improvement in seizing resistance cannot be obtained by agraphite content less than 0.5%. A graphite content exceeding 5%deteriorates the mechanical strength of the resulting roll extremely.The preferred graphite content is 2-4%. The particle size of thegraphite particles is 5-50 μm.

(2) To improve wear resistance, it is required that the hard carbidesare well dispersed. To this end, MC carbides should be contained in anarea ratio of 0.2-1%. Only insufficient wear resistance can be obtainedby an MC content less than 0.2%. It is practically unable to contain theMC carbides exceeding 10% by area ratio due to the coexistence of thegraphites. The preferred content of the MC carbides is 4-8%.

(3) Since the cementite, which is one of soft carbides, shows a littleeffect for improving wear resistance, the amount of the cementite ispreferred to be minimized. However, the cementite and the graphite aregenerated in nearly the same condition. Therefore, it is impossible toallow the graphites only to crystallize without accompanied by thegeneration of cementite. When the content of the cementite exceeds 40%by area ratio, the toughness of the roll is deteriorated. The preferredarea ratio of cementite is 1-30%.

(4) The roll may contain at east one of M₂ C carbides, M₆ C carbides andM₇ C₃ carbides in an tea ratio of 0.2-20% in addition to the MCcarbides. An area ratio less than 0.2% provides no sufficient effect,whereas an area ratio exceeding 20% deteriorates the toughness of theroll because the area ratio of the total carbides including thecementite becomes too large. The preferred area ratio of the carbidesexcluding the MC carbides is 4-15%.

(5) The matrix of the roll is preferred to substantially comprisemartensite, bainite or pearlite.

(b) Composition

In order to meet the above structural requirements, the wear- andseizing-resistant roll for lot rolling of the present invention has thefollowing composition.

(1) C: 2.0.-4.0 weight %

C is an indispensable element for forming hard carbides by bonding withthe coexisting elements of Cr, V, Mo and W to enhance wear resistance aswell as for crystallizing graphite particles to impart seizing resistantto the roll. When the content of C is less than 2.0 weight %, the amountof the hard carbides is too small and the graphite particles hardlycrystallize. When the content of C is more than 4.0 weight %, the amountof the cementite and the hard carbides are too large to deteriorate thetoughness of the roll. The preferred content of is 2.5-3.5 weight %, andmore preferred content is 2.8-3.2 weight %.

(2) Si: 0.5-4.0 weight %

Si is a graphitizing element and is necessary to be contained in anamount of 0.5 weight % or more. When the content exceeds 4.0 weight %,the matrix of the resulting roll becomes brittle to decrease thetoughness. In addition, Si is necessary to be inoculated in an amount of0.1 weight % or more, preferably 0.1-0.8 weight % for crystallizing thegraphites in a suitable amount. The Si content mentioned above means thetotal content of the Si originally contained in a melt of roll materialand the Si inoculated into the melt. The total content of Si in the rollis preferably 0.8-3.5 weight %, and more preferably 1.5-2.5 weight %.

(3) Mn: 0.1-1.5 weight %

Mn has a function of deoxidizing a melt and fixing S contained as animpurity, and is necessary to be contained in an amount 0.1 weight % ormore. When the content exceeds 1.5 weight %, retained austenite tends tobe generated, making it difficult to maintain sufficient hardness. Thepreferred content of Mn is 0.2-1.0 weight %, and more preferred contentis 0.3-0.6 weight %.

(4) Cr: 1.0-7.0 weight %

Cr is effective for maintaining sufficient hardness and wear resistanceby generating bainite matrix or martensite matrix, and is necessary tobe contained 1.0 weight % or more. When Cr is contained in excessivelylarge amount, the crystallization of graphite is inhibited or theroughness of the matrix becomes low, as well as, Cr carbides such as M₇C₃ and M₂₃ C₆ are generated. Such Cr carbides are lower than MC carbideor M₂ C carbides in hardness, so that improvement in wear resistancecannot be expected and the resulting roll becomes brittle. Therefore,the upper limit of the Cr content is 7.0 weight %. The preferred contentis 1.0-5.0 weight %, and more preferably 1.5-3.0 weight %.

(5) Mo: 2.0-10.0 weight %

Mo is effective for increasing wear resistance because Mo forms hard M₆C, M₂ C carbides by bonding with C, and further, strengthens the matrixby dissolving thereinto. On the other hand, an excess Mo tends toinhibit the crystallization of graphite because Mo is a white castiron-forming element. Therefore, the Mo content is 2.0-10 weight %,preferably 2.0-8.0 weight %, and more preferably 3.0-6.0 weight %.

(6) V: 2.0-8.0 weight %

V forms MC carbides by with C. This MC carbide have a Vickers hardnessof 2500-3000 and is the hardest one among the carbides. Therefore, isthe most effective, indispensable element for increasing wearresistance. However, an excess V inhibits the crystallization fgraphite. Accordingly, the V content is 2.0-8.0 weight %, preferably2.0-6.0 weight %, and more preferably 3.0-6.0 weight %.

(7) Ni: 0.2-4.0 weight %

In addition to the indispensable elements described above, the roll ofthe present lion may further contain Ni. Ni has functions to promote thecrystallization of graphite and to improve the hardenability of the m,However, Ni shows no such functions when the Ni content is less than 0.2weight %. On the other hand, when the content exceeds 4.0 weight %, theaustenite is stabilized too much to make it difficult to transform intobainite or martensite. The preferred Ni content is 0.5-2.0 weight %.

(8) W: 2.0-10.0 weight %

In addition to the indispensable elements described above, the roll ofthe present invention may further contain W. W is effective forincreasing wear resistance because W like Mo forms hard M₆ C, M₂ Ccarbides by bonding with C, and further, strengthens the matrix bydissolving thereinto. )n the other hand, an excess W tends to inhibitthe crystallization of graphite because W is a white cast iron-formingelement. Therefore, the preferred W content is 2.0-10 weight %, and morecontent is 2.0-6.0 weight %.

(9) Co: 1.0-10.0 weight %

In addition to the indispensable elements described above, the roll ofthe present invent on may further contain Co. Although Co is effectivefor strengthening the matrix, an excess Co tends to decrease thetoughness. Therefore, the Co content is 1.0-10.0 weight %. Co furtherhas a function to make cementite instable to promote the crystallizationof graphite. The preferred Co content is 3.0-7.0 weight %.

(10) Nb: 1.0-10.0 weight %

In addition to the indispensable elements described above, the roll ofthe present invention may further contain Nb. Nb like V forms MCcarbides by bonding with C. Since this MC carbide, as described above,is the hardest me among the carbides, Nb is the most effective elementfor increasing wear resistance. However, an excess Nb inhibits thecrystallization of graphite. Accordingly, the Nb content is preferably1.0-10.0 weight %, and more preferably 2.0-6.0 weight %.

(11) Ti: 0.01-2.0 weight %

In addition to the indispensable elements described above, the roll ofthe present invention may further contain Ti. Ti forms oxy-nitrides bywith N and O which are anti-graphitizing elements. Ti less than 0.01weight % shows no such effect, and Ti up to 2.0 weight % is sufficientfor the purpose in consideration of the contents of N and O. The morepreferred Ti content is 0.05-0.5 weight %.

(12) B: 0.002-0.2 weight %

In addition to the indispensable elements described above, the roll ofthe present invention may further contain B. Although B has a functionto make he carbides fine, B less than 0.002 weight % shows such functioninsufficiently. On the other hand, B exceeding 0.2 weight % ma thecarbides instable. Accordingly, the B content is preferably 0.002-0.2weight %, and more preferably 0.01-0.05 weight %

(13) Cu: 0.02-1.0 weight %

In addition to the indispensable elements described above, the roll ofthe present invention may further contain Cu. Cu like Co has a functionto make cementite instable to promote the crystallization of graphite.Cu less than 0.02 weight % shows insufficient effect, whereas Cu lessthan 1.0 weight % results in reduced toughness. Accordingly, the Cucontent is preferably 0.02-1.0 weight %, and more preferably 0 1-0.5weight %.

(14) Balance

Beside the above elements, the roll consists substantially of Fe exceptfor impurities. Major impurities are P and S, and it is preferred that Pis 0.1 weight % or less and S is 0.08 weight % or less for preventingthe toughness from decreasing.

2! Wear- and seizing-resistant compound roll for hot rolling

The wear- and seizing-resistant roll for hot rolling of the presentinvention may be a compound roll. The outer layer of the compound rollis made of the alloy having the metal structure and the composition,both described above. The shaft of the compound roll, which bondsmetallurgically to the outer layer, is made of steel including caststeel and forged steel. It is preferable that the shaft has a tensilestrength of 55 kg/mm² or more and an elongation of 1.0% or more. This isbecause when used for rolling, the shaft is subjected to large rollingforce, and a bending force is applied both ends of the shaft tocompensate the deflection of the roll during the rolling operation, sothe shaft should withstand such rolling force bending force.

In addition, the shaft should be strongly bonded to the outer layer madeof the above iron-based alloy. Accordingly, the bonding strength of theouter la, interface should be higher than or equal to the mechanicalstrength of weaker one of the outer layer and the shaft.

3! Production of the wear- and seizing-resistant roll for hot rolling

Since the material for he roll of the present invention is high speedsteel, the roll is to be produced into a compound roll by a centrifugalcasing method or a shell casting method. In both the casting methods, anSi-containing inoculant should be added to a melt having he abovecomposition. Although the inoculating amount of Si is at least 0.1weight %, the inoculant becomes difficult to dissolve in a meltuniformly when the inoculating amount of Si exceeds 0.8 weight %,resulting in uneven metal structure of the resulting outer layer.

The production of a compound roll is exemplified below in the shellcasting method.

The shell casting method is basically disclosed in WO 88/07594. FIG. 1shows an example of an apparatus for use in continuous shell castingmethod. This apparatus comprises a composite mold 10 comprising atunnel-shaped refractory mold 1 having a tapered portion and acylindrical portion and a cooling mold 4 provided under andconcentrically with the refractory mold.

The refractory mold 1 is surrounded by an annular induction heating coil2, and a lower end of the refractory mold 1 is provided with aconcentric, annular buffer mold 3 having the same inner diameter as thatof the refractory, mold 1. Attached to a lower end of the buffer mold 3is a cooling mold 4 having substantially the same inner diameter as thatof the buffer mold 3. Cooling water is introduced into the cooling mold4 through an inlet 14 and discharged through an outlet 14'.

A roll shaft 5 is inserted into a composite mold 10 having the abovestructure. The shaft 5 is provided with a closure member (not shown)having substantially the same diameter as that of an outer layer to beformed at a lower end of the shaft or at a position appropriatelyseparate from the lower end of the shaft. The lower end of the shaft 5is mounted to a vertical movement mechanism (not shown). A melt 7 isintroduced into a space between the shaft 5 and the refractory mold 1,and a surface of the melt 7 is sealed against the air by a melted flux6. The melt 7 is stirred by convection in the direction shown by thearrow A in FIG. 1. Next, the shaft 5 is gradually moved downwardtogether with the closure member fixed thereto. Due to the downwardmovement of the shaft and the closure member the melt 7 is lowered andbegins to be solidified when contacted with the buffer mold 3 and thecooling mold 4. By this solidification, the shaft and the outer layerare completely metallurgically The surface of the melt held in therefractory mold 1 is also lowered together with the descent of the shaft5 and the close member, but a fresh melt is appropriately supplied tokeep the melt surface at a certain level. By successively repeating thedescent of the shaft 5 and pouring of the melt 7, the melt 7 isgradually solidified from below to form an outer layer 8.

During the above continuous casting, an Si-containing inoculant isinjected into the melt 7 held in the refractory mold 1. Ca--Si alloy ispreferably used as the Si-containing inoculant while a sufficientgraphite crystallization cannot be attained by Fe--Si alloy. The Sicontent in the Ca--Si alloy is 55-65 weight %.

The inoculant should be injected just before the initiation ofsolidification of the melt because the duration of the inoculatingeffect is only about 5 minutes. Therefore, the inoculation by merelymixing the inoculant with the melt 7 or ladle inoculation is notemployed, but inoculation is conducted by injecting a wire 16 containingthe inoculant into the portion as close to the solidifying portion ofthe met as possible. With this so-called wire-injection method, theresulting solidified outer layer 8 contains a sufficient amount ofcrystallized graphite particles.

The wire 16 containing the inoculant is preferred to be made of mildsteel for avoiding the change of the composition of the outer layer. Thewire 16 is of having an outer diameter of about 6-14 mm and an inner of5.6-13 mm, and the inner space of the wire is filled with theSi-containing inoculant. The wire 16 made of mild steel is fused in themelt 7 to allow the Si-containing inoculant contained therein to beexposed and fused in the melt thereby inoculating Si. For effectivelyinoculating Si, the tip of the wire 16 is kept at the vicinity of thesurface of solidifying melt.

The compound roll thus prepared is further subjected to heat treatmentsuch as hardening and tempering according to known methods.

The present invention will be explained in further detail by means ofthe following Examples.

EXAMPLE 1

Each melt of 1550° C. having a composition shown in Table 1 was pouredat a pouring temperature of 1400° C. into a sand mold of 100 mm diameterand 100 mm depth containing a Ca--Si alloy inoculant in 0.2 weight %.The cast product was subjected to hardening from 1100° C. andsubsequently to tempering at 550° C. repeatedly three times to prepareeach test roll. In Table 1, the test rolls Nos. 1-7 are within thepresent invention, the test roll No. 8 is made of a grain cast steel,and the test roll No. 9 is made of a high speed steel with no of Si.Microphotographs of the metal structures at the position 50 mm distantfrom the bottom of the test roll No. 2 are shown in FIGS. 2-4.Specifically, FIG. 2 shows the metal structure of the surface subjectedto diamond polishing. In FIG. 2, the black portion is graphite particlesand the white ground portion is carbides and matrix. FIG. 3 shows themetal structure of the surface subjected to etching with picric acid.The etching treatment made it possible observe the structures oftempered bainite matrix, martensite x and carbides. FIG. 4 shows themetal structure of the surface subjected to electrolytic etching withchromic acid. By the electrolytic etching with chromic acid, MC carbidescame possible to observed as black portion which also includes graphiteparticle: All the carbides (MC carbides, M₂ C carbides, M₆ C cementite,etc.) can be observed by etching with a solution of ammonium persulfate.The area ratios of the graphite and carbides were measured by an imageanalyzer (manufactured by Avionics Co. Ltd.). The results are shown inTable 2.

                  TABLE 1    ______________________________________    (weight %)    ______________________________________    Test roll No.              C      Si     Mn    Ni   Cr   Mo    V    ______________________________________    1         2.9    1.9    0.5   1.0  2.8  3.1   4.5    2         3.0    2.0    0.5   0.9  3.0  2.9   4.5    3         3.1    2.0    0.5   1.2  3.1  2.5   4.0    4         3.3    2.7    0.4   0.8  2.7  3.3   3.0    5         3.0    2.0    0.5   0.8  2.3  2.2   3.8    6         2.9    1.8    0.5   0.9  2.5  2.1   4.5    7         3.0    2.0    0.5   0.9  2.2  4.3   4.4      8.sup.(1)              3.1    1.0    0.7   4.5  1.8  0.3   --      9.sup.(2)              2.1    0.8    0.4   0.5  6.2  3.5   5.9    ______________________________________    Test roll No.              W      Co     Nb    Ti   B    Cu    ______________________________________    1         --     --     --    --   --   --    2         2.2    --     --    --   --   --    3         2.1    5.2    --    --   --   --    4         --     --     2.3   --   --   --    5         3.1    --     --    0.5  --   --    6         2.5    --     --    --   0.05 --    7         3.0    --     --    --   --   0.2      8.sup.(1)              --     --     --    --   --   --      9.sup.(2)              2.2    --     --    --   --   --    ______________________________________     Note:     .sup.(1) Grain roll     .sup.(2) High speed steel roll

EXAMPLE 2

Small sleeve rolls of 60 mm outer layer, 40 mm inner layer and 40 mmwidth prepared from the test rolls Nos. 2 and 5 were subjected torolling wear test using the rolling wear test apparatus shown in FIG. 5and seizing test using the frictional-heat shock test apparatus shown inFIG. 6. Further, the same tests were conducted on the sleeve rollsformed, from the test roll No. 8 (grain roll) and test roll No. 9 (highspeed steel roll). Wear resistance of each roll was evaluated by thewear depth after repeating the test three times.

The rolling wear test apparatus comprises a rolling mill 21, an upperroll 22 and a lower roll 23 in the rolling mill 21, a heating furnace 24for preheating a sheet S to be rolled, a cooling water bath 25 forcooling the rolled S, a reel 26 for giving a constant tension to thesheet during rolling operation, and a tension controller 27 foradjusting the tension. The test conditions were as follows:

Sheet to be rolled: SUS304 1 mm thick and 15 mm wide

Rolling reduction: 25%

Rolling speed: 150 m/minute

Rolling temperature: 900° C.

Rolling distance: 300 m

Roll cooling: Water cooling

Number of rolls: Four

In the frictional-heat shock test apparatus shown in FIG. 6, a weight 39is allowed to fall onto a rack 38 to rotate a pinion 30, therebybringing a biting member Into strong contact with the surface of a testpiece 31.

The results are shown in Table 2. The wear depth of each roll of thepresent invention as about 1/4 of that of the grain cast iron roll, andwas equal to that of the high speed steel roll. With respect to theseizing area ratio, the ratio of each roll of the present invention wasnearly the same as that of the grain cast iron roll, and about 60% ofthat of the high speed steel roll. These results show that seizingresistance increases with increasing graphite amount.

As mentioned above, the roll of the present invention is comparable tothe conventional cast iron roll in seizing resistance, and is 4 timeshigher than it in wear resistance. Further, the roll of the presentinvention shows more improved seizing resistance as compared with thehigh speed steel roll having little graphite.

                  TABLE 2    ______________________________________             Area ratio  Area ratio  Area ratio    Test     of Graph-   of MC car-  of Car-    roll No. ite (%)     bides (%)   bide (%)    ______________________________________    2        2.7         5.5         24.1    5        2.2         4.7         23.8      8.sup.(1)             2.5         --          38.6      9.sup.(2)             --          7.3         20.7    ______________________________________             Ratio of    Wear    Test     seizing     depth    roll No. area (%)    (μm)    ______________________________________    2        41          6    5        40          7      8.sup.(1)             38          27      9.sup.(2)             63          7    ______________________________________     Note:     .sup.(1) Grain roll     .sup.(2) High speed steel roll

EXAMPLE 3

By using melt having same composition as test roll No. 2 in Example 1, acompound roll of 600 mm outer diameter and 1800 mm roll length wasproduced by the continuous shell casting apparatus shown in FIG. 1. Themelt temperature was 1580° C. and the pouring temperature was 1350° C. ACa--Si inoculant was injected, as shown in FIG. 1, into melt held in therefractory mold 1 by wire injection method. S i amount inoculated was0.2 weight %. The compound roll thus ed was subjected to stress reliefannealing, hardening from 1100° C., and then tempering three times at550° C. for 20 hours.

The compositions of the outer layer at 5 mm depth, 25 mm depth and 50 mmdepth of the upper casting portion, mid casting portion and lowercasting portion of the roll barrel were examined. The results are shownin Table 3. Further, the observation of metal structures of the sameportions as above showed the results of 2.0-3.0% of graphite area ratio,4.5-5.5% of MC carbides area ratio and 20-25% if the total carbidesratio (MC carbide, M₂ C carbides, M₆ C carbide and cementite). Theseresults are nearly the same as those of Example 1, and demonstrates theexcellency of the compound roll of the present invention in wearresistance and seizing resistance.

                  TABLE 3    ______________________________________    (weight %)    ______________________________________    Portion        C       Si       Mn    Ni    ______________________________________    Upper casting portion     5 mm          3.04    2.10     0.48  0.90    25 mm          3.00    2.08     0.47  0.88    50 mm          2.98    2.08     0.47  0.88    Mid casting portion     5 mm          2.99    1.96     0.48  0.91    25 mm          3.05    1.98     0.49  0.87    50 mm          3.02    1.98     0.50  0.88    Lower casting portion     5 mm          3.01    1.92     0.51  0.90    25 mm          2.99    1.88     0.48  0.91    50 mm          2.99    1.91     0.47  0.95    ______________________________________    Portion        Cr      Mo       V     W    ______________________________________    Upper casting portion     5 mm          2.85    2.89     4.44  2.20    25 mm          2.91    2.90     4.48  2.17    50 mm          2.95    2.90     4.47  2.11    Mid casting portion     5 mm          2.90    2.81     4.45  2.18    25 mm          3.02    2.85     4.46  2.08    50 mm          2.96    2.86     4.48  2.16    Lower casting portion     5 mm          2.84    2.85     4.51  2.22    25 mm          2.93    2.78     4.53  2.19    50 mm          2.95    2.77     4.51  2.26    ______________________________________

INDUSTRIAL APPLICABILITY

By coexisting graphite articles and hard carbides, it has been madepossible to provide rolls for hot rolling having both wear resistanceand seizing resistance. Such rolls are highly efficient particularlyused in the latter stand of a finishing train of a hot strip mill. Withsuch rolls, the productivity in the rolling manufacture can beincreased.

We claim:
 1. A wear- and seizing-resistant roll for hot rolling, whichhas a composition consisting essentially, by weight, of 2.0-4.0% of C,0.5-4.0% of Si, 0.1-1.5% of Mn, 1.0-7.0% of Cr, 2.0-10.0% of Mo,2.0-8.0% of V and balance of Fe and inevitable impurities, and has ametal structure comprising a matrix, 0.5-5% in area ratio of graphite,0.2-10% in area ratio of MC carbides and 40% or less in area ratio ofcementite.
 2. The wear- and seizing-resistant roll for hot rollingaccording to claim 1, wherein said metal structure further contains inaddition to said MC carbides at least one carbide of M₂ C carbides, M₆ Ccarbides and M₇ C₃ carbides in an area ratio of 0.2-20%.
 3. The wear-and seizing-resistant roll for hot rolling according to claim 1 or 2,wherein said matrix substantially comprises martensite, bainite orpearlite.
 4. The wear- and seizing-resistant roll for hot rollingaccording to any one of claims 1-3, wherein said composition furtherconsists essentially, by weight, of at least one of 0.2-4.0% of Ni,2.0-10.0% of W, 1.0-10.0% of Co, 1.0-10.0% of Nb, 0.01-2.0% of Ti,0.002-0.2% of B and 0.02-1.0% of Cu.
 5. The wear- and seizing-resistantroll for hot rolling according to any one of claims 1-3, wherein saidroll has a composition consisting essentially, by weight, of 2.0-4.0% ofC, 0.5-4.0% of Si, 0.1-1.5% of Mn, 1.0-7.0% of Cr, 2.0-10.0% of Mo,2.0-8.0% of V, 0.2-4.0% of Ni, 2.0-10.0% of W, balance of Fe andinevitable impurities, and at least one of 1.0-10.0% of Co, 1.0-10.0% ofNb, 0.01-2.0% of Ti, 0.002-0.2% of B and 0.02-1.0% of Cu.
 6. A wear- andseizing-resistant compound roll for hot rolling, which comprises anouter layer of a wear- and seizing-resistant iron-based alloy and asteel shaft metallurgically bonded to the outer layer, the iron-basedalloy having a composition consisting essentially, by weight, of2.0-4.0% of C, 0.5-4.0% of Si, 0.1-1.5% of Mn, 1.0-7.0% of Cr, 2.0-10.0%of Mo, 2.0-8.0% of V and balance of Fe and inevitable impurities, andhaving a metal structure comprising a matrix, 0.5-5% in area ratio ofgraphite, 0.2-10% in area ratio of MC carbides and 40% or less in arearatio of cementite.
 7. The wear- and seizing-resistant compound roll forhot rolling according to claim 6, wherein said metal structure of saidouter layer further contains in addition to said MC carbides at leastone carbide of M₂ C carbides, M₆ C carbides and M₇ C₃ carbides in anarea ratio of 0.2-20%.
 8. The wear- and seizing-resistant compound rollfor hot rolling according to claim 6 or 7, said matrix of said outerlayer substantially comprises martensite, bainite or pearlite.
 9. Thewear- and seizing-resistant compound roll for hot rolling according toany one of claims 6-8, wherein said iron-based alloy of said outer layerfurther contains by weight, of at least one of 0.2-4.0% of Ni, 2.0-10.0%of W, 1.0-10.0% of Co, 1.0-10.0% of Nb, 0.01-2.0% of Ti, 0.002-0.2% of Band 0.02-1.0% of Cu.
 10. The wear- and seizing-resistant compound rollfor hot rolling according to any one of claims 6-9, wherein said outerlayer has a composition consisting essentially, by weight, of 2.0-4.0%of C, 0.5-4.0% of Si, 0.1-1.5% of Mn, 1.0-7.0% of Cr, 2.0-10.0% of Mo,2.0-8.0% of V, 0.2-4.0% of Ni, 2.0-10.0% of W, balance of Fe andinevitable impurities, and at least one of 1.0-10.0% of Co, 1.0-10.0% ofNb, 0.01-2.0% of Ti, 0.002-0.2% of B and 0.02-1.0% of Cu.
 11. A methodof producing a wear- and seizing-resistant compound roll for hotrolling, which comprises an outer layer of a wear- and seizing-resistantiron-based alloy and a steel shaft metallurgically bonded to the outerlayer, the iron-based alloy having a composition consisting essentially,by weight, of 2.0-4.0% of C, 0.5-4.0% of Si, 0.1-1.5% of Mn, 1.0-7.0% ofCr, 2.0-10.0% of Mo, 2.0-8.0% of V and balance of Fe and inevitableimpurities, and having a metal structure comprisingly a matrix, 0.5-5%in area ratio of graphite, 0.2-10% in area ratio of MC carbides and 40%or less in area ratio of cementite, wherein in Si-containing inoculantis supplied into a melt of material for said outer layer at least in avicinity of a bonding portion of said melt and said steel shaft.
 12. Themethod of producing a wear- and seizing-resistant compound roll for hotrolling according to claim 11, wherein said Si-containing inoculant isinjected into the vicinity of the bonding portion of said melt and saidsteel shaft by means of wire-injection method.
 13. The method ofproducing a wear- and seizing-resistant compound roll for hot rollingaccording to claim 11 or 12, wherein said method comprises the stepsof:introducing said steel shaft concentrically into an inner space of acomposite mold comprising a refractory mold surrounded by an inductionheating coil and a cooling mold provided under said refractory moldconcentrically therewith; pouring said melt of the iron-based alloy intoa space between said steel shaft and said composite mold; keeping saidmelt at a temperature between a primary crystal-crystallizingtemperature and a temperature 100° C. higher than said primarycrystal-crystallizing temperature heating with stirring while sealingthe surface of said melt by a flux; moving said steel shaft downwardconcentrically with said composite mold to bring said melt into contactwith said cooling mold thereby solidifying said melt to bond to saidsteel shaft so that said outer layer is continuously formed on saidsteel shaft body, during the formation of said layer an Si-containinginoculant being injected by means of wire-injection method into saidvicinity of the bonding portion said melt and said steel shaft tocrystallize graphite particles in a amount.
 14. The method of producinga wear- and seizing-resistant compound roll for hot rolling according toany one of claims 11-13, wherein said Si-containing inoculant is Ca--Sialloy.